The promise of drag reduction over solid surfaces in high Reynolds number flows is one that has captured the attention of researchers for years, yet has remained illusive. In the past, numerous approaches have used both passive and active methods to control the flow in a turbulent boundary layer. In one exemplary approach, it is relatively well known that the aerodynamic drag of a surface may be reduced by applying a microscopic “texture” to the otherwise smooth surface. Although the exact fluid dynamic mechanism at work in this drag reduction is not well understood, it is speculated that the reduction relates to controlling the turbulent vortices in the boundary layer adjacent to the surface. The microscopic texture reduces the skin friction drag of solids moving through fluids (e.g., aircraft, ships, cars, etc.), and of fluids moving along solids (e.g., pipe flow, etc.).
One well known geometric form for a microscopic, friction-reducing texture is known as “riblets.” Conventionally, riblets are positioned on a surface to form an integrated series of groove-like peaks and valleys with V-shaped cross-sections. Normally, the riblets are positioned to extend along the aerodynamic surface of the object in the direction of fluid flow. In one example, the height of the riblets and the spacing between the riblets are usually uniform and on the order of 0.001 to 0.01 inches for most applications.
Dimensionless units, sometimes referred to as wall units, are conventionally utilized in describing fluid flows of this type. The wall unit h+ is the non-dimensional distance away from the wetted surface or more precisely in the direction normal to the surface, extending into the fluid. Thus h+ is a non-dimensional measurement of the height of the riblets. The wall unit s+ is the non-dimensional distance tangent to the local surface and perpendicular to the flow direction, thus the non-dimensional distance between the riblets. In the prior art riblets, h+ and s+ are in the range between 10 and 20. Exemplary riblet designs can comprise an adhesive film applied to a smooth solid surface or alternatively, with advanced manufacturing techniques, the same shapes may be directly formed and integrated into the structure of the aerodynamic surface.
The interaction of riblets with the structure of the turbulent boundary layer of the fluid reduces the skin friction drag coefficient (Cdf) of the surface by approximately 6% compared to an identical smooth surface without riblets. This reduction occurs despite the significant increase in “wetted area” (the surface area exposed to the fluid stream) of a riblet-covered surface over a smooth surface. In attempts to further reduce the Cdf, modifications to conventional V-shaped riblets have been proposed. Examples include rounding of the peaks and/or valleys of the respective riblets, as well as even smaller V-shaped notches in the sides of the larger V-shaped riblets.
Further examples of improved riblet designs that decreases skin friction drag with less concomitant increase in wetted area than conventional riblets include the use of a series of parallel riblets that extend longitudinally from a smooth surface. In this example, the riblets have a triangular cross-section in the transverse direction in which the apex of the cross-section defines a continuous, undulated ridge with peaks and valleys that causes an effective reduction in Cdf. The wetted area of this exemplary design is increased less than with conventional riblets.